how to find the average speed of a motor?
A 1hp 250 rpm dc motor has a rated torque at 40 Nm. The motor constants are armature resistance = 5 ohms and the motor constant 7.5 V s/r or 7.5 Nm/A. the motor is operating at 45% of rated torque in a single phase half wave phase controlled converter. the firing angle is 90 degrees and the extinction angle is 210 degrees. the rms value of the line voltage is 240V, find the average speed of the motor with 1hp = 746Watts.
Before you can solve this problem, you need to find the correct statement of the problem. A 1 hp (.746 kW), 250 rpm motor has a rated torque of 28.5 N-m. I don’t know why the problem states that the rated torque is 40 N-m.
You need to calculate the average motor voltage over the conduction period by integrating the sine wave from the firing point to the extinction point. I believe that will be about 140 volts.
Then calculate the average armature current for 45% of rated torque from current = torque / motor constant.
Since the conduction period is 120 degrees or 1/3 of the total period of the complete waveform, the average armature current over the conduction period must be three times the average current required to produce 45% of rated torque.
Then calculate the average speed as the average voltage generated by the armature (back emf) divided by the motor constant. The average back emf is the average motor voltage minus the IR drop, average armature current X armature resistance.
8 qt & 3.5 qt Vertical Cutter Mixer, 2 Speed, 208-240V, 3-ph
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